There isn't anyway to keep the add panel open once you drag your chosen element into the design. I imagine this is to account for users with smaller screens, or for people who are editing things on the left side of the screen.

You can toggle the add panel on and off by clicking "a"You can also see all of the shortcuts available to you by clicking "?". For me it is actually "shift + /"

I'd prefer to be able to keep it open all the time.Maybe a "lock" button to keep the panel open so that users can choose to keep open or not.That way everyone can set it the way that works best for them.